1. Field of the Invention
The present invention relates to a delay circuit, a jitter injection circuit, and a test apparatus. More particularly, the present invention relates to a jitter injection circuit capable of injecting a jitter with high frequency and large amplitude to an input signal.
2. Related Art
As a circuit injecting a jitter into an input signal, a circuit making use of a variable delay element is considered as disclosed in Japanese Patent Application Publication No. 2006-41640. For example, it is possible to inject a jitter into an input signal by including a variable delay element for delaying the input signal and a control section for controlling a delay amount in the variable delay element and changing the delay amount in the variable delay element according to the jitter to be injected.
Moreover, jitter amplitude capable of being injected is determined by variable delay range of the variable delay element. For this reason, when injecting a jitter of large amplitude, it is possible to realize a large variable delay range as a whole circuit by cascading variable delay elements, each of which a split delay-control signal is fed into.
For example, when a sinusoidal jitter of large amplitude is injected into the input signal, a time delay of each variable delay element is varied from a predetermined reference time delay according to the sinusoid. A period of the delay control signal to be split and fed into each variable delay element corresponds to a period of the sinusoidal jitter to be injected.
When each variable delay element generally uniformly delays a predetermined edge of the input signal according to the jitter to be injected, it is possible to add the time delays in the variable delay elements and inject the sinusoidal jitter of large amplitude into the input signal.
Here, when a time delay generated from one variable delay element is sufficiently small in comparison with the period of the delay control signal, a variation in time delay of the delay control signal, which is required for a predetermined edge of the input signal during the time interval between being input into a variable delay element and being output into the next variable delay element, is also sufficiently small. In this case, it can be considered that a level of the delay control signal given to each variable delay element is a generally same level according to amplitude of the jitter to be injected. For this reason, it is possible to inject the jitter of large amplitude into the input signal.
On the other hand, when a time delay generated from one variable delay element is not sufficiently small in comparison with the period of the delay control signal, an amplitude of the delay control signal is varied while the input signal propagates trough one single stage in the variable delay elements. For this reason, it is difficult to inject a jitter with desired amplitude into the input signal. Particularly, when injecting a jitter with high jitter frequency, a time delay injected in a positive direction with respect to the reference time delay and a time delay injected in a negative direction with respect to the reference time delay may be cancelled with each other, and thus it is difficult to inject a jitter with high jitter frequency and large amplitude.
Moreover, when a jitter test is performed on a device under test such as a semiconductor circuit by means of the jitter injection circuit, it is impossible to perform the test with high precision.